Jakob Isaak was born 8 October 1900 in Lindenau, Molotschna Colony, Russia, to Gerhard Isaak and Maria Mantler. He was baptized 17 May 1920, and joined the Mennonite church (Kirchen-Gemeinden). He married Elisabeth Hildebrandt, whose parents were Gerhard Hildebrandt and Elisabeth Goossen on 5 September 1924. Jakob and Elisabeth became the parents of 12 children, including Kornelius Isaak. The Isaak family was able to leave Russia in 1929 and settled in the village of Auhagen in Fernheim Colony, Paraguay. Jakob Isaak was ordained a minister on 28 October 1934, assumed responsibility for the congregation in 1944, and served as elder, 1948-1970.

Elder Isaak's two great concerns were the strengthening of spiritual life among the congregations and the evangelizing of the Chaco Indians (Chaco mission). Youth work was formally organized in 1940 and a choir in 1945. A new church building was dedicated in Filadelfia in 1950. Once a month the entire congregation met for Bible study, to give testimony, and to receive instruction. The relationship of the Mennonite church (CGM) to other congregations and conferences (Mennonite Brethren, Evangelische Mennonitische Brüderschaft) was particularly cordial because of Jakob Isaak's work. He was chairman of the Kommission für Kirchenangelegenheiten (Commission for Church Affairs) for 16 years, of Licht den Indianern (Light to the Indians), the Vermittlungskomitee, and the Conference of Mennonites in South America. He died 12 October 1981.